In U.S. Pat. No. 4,751,613 to Werdin and Henke, issued June 14, 1988, there is disclosed a low RF emission fiber optic transmission system. In that system there is provided a conversion unit which converts electrical signals to fiber optic signals and vice versa. The system can thus provide a secure fiber optic link between electrical systems. In addition, the conversion unit is designed to suppress and contain RF emissions from the conversion circuitry.
The system described in U.S. Pat. No. 4,751,613 is modular and permits specialized conversion modules to be mixed and matched. Modules for multiplexing and demultiplexing RS232-type data channels, test modules, power supply modules, high speed data conversion modules, voice frequency multiplexing modules and telephone interfacing modules are all contemplated by the system.
The present application discloses a modification to the conversion unit described in U.S. Pat. No. 4,751,613, together with certain additional new circuits and circuitry features for use in the conversion electronics thereof. More particularly, there is disclosed a conversion unit having front and rear modularity, a design methodology for implementing bypass capacitors to minimize RF emissions from the power lead of circuit components, a loop current regulating circuit for a PBX module and a voice frequency multiplexing circuit and method.